1. Field of the Invention
The present invention relates to protection of an inverter apparatus for system interconnection.
2. Description of the Related Art
A system for converting DC power such as a solar cell or a fuel cell into AC power using an inverter and connecting the converted power to a power source system through a coupling reactor or a transformer has become popular in view of effective use of energy. As described in Section 9.3 in "Semiconductor Power Conversion System", Institute of Electrical Engineers of Japan, the system coupling inverter is largely classified into a line-commutated type inverter for performing an inverter operation by utilizing an AC voltage and a self-commutated type inverter capable of desirably controlling an output voltage phase of an inverter. The self-commutated inverter can desirably supply delayed or advanced reactive power to an AC power system, and can be stably operated even if an AC power system capacity is small.
In a conventional self-commutated apparatus, an inverter outputs inverter voltage VI having phase .theta.I with respect to AC power system voltage Vs having phase .theta.s, and controls reactive power and active power. FIGS. 3A and 3B show vector charts of the voltage and phase. FIG. 3A shows an operation state of power factor=1 wherein phases of system voltage Vs and output current I coincide with each other. FIG. 3B shows a case wherein current I advanced from system voltage Vs is supplied. Inverter voltage VI corresponds to a vector sum of system voltage Vs and voltage XI corresponding to a voltage drop across a coupling reactor whose phase is advanced by 90.degree. from current I.
When the power factor is 1, as shown in FIG. 3A, Vs and I are in phase, and inverter phase .theta.I is advanced from a system voltage phase by a value substantially proportional to current I. When advanced current I shown in FIG. 3B is supplied, the magnitude of voltage VI can be decreased by reactive power corresponding to current I. In contrast to this, when delayed current I is supplied, voltage VI is increased. Therefore, active power can be controlled by changing .theta.I, and reactive power can be controlled by controlling the magnitude of voltage VI.
With the above arrangement, a system coupling generator capable of controlling active and reactive powers can be realized.
In the conventional arrangement, when an AC circuit breaker is opened and a service interruption in an AC system occurs, the inverter is preferably stopped as soon as possible. In an inverter of this type, when a frequency or voltage is deviated from a standard value, an abnormal voltage or frequency detector operates to stop the inverter. (In this case, a load is present nearer to the inverter than the AC circuit breaker.) When the AC circuit breaker is opened, phase angle .theta.I defined by inverter output voltage VI and system voltage Vs is determined by the magnitude of the load. When inverter output active power is different from load active power before the AC circuit breaker is opened, .theta.I varies before and after circuit opening. A frequency changes by a value obtained by integrating the difference, and a change in frequency is detected as an abnormal frequency. As a result, the inverter is stopped.
When the inverter output reactive power is different from the load reactive power, the magnitude of system voltage Vs varies before and after the AC circuit breaker is opened, and reactive power also varies.
For these reasons, if system voltage Vs is decreased, inverter voltage reference VI* is decreased, and the inverter decreases output voltage VI in response to reference VI*. With this positive feedback operation, system voltage Vs is kept decreased, and an abnormal voltage of the inverter is then detected. As a result, the operation of the inverter is stopped.
In contrast to this, if system voltage Vs is increased, inverter voltage reference VI* is increased, and the inverter increases its output voltage VI in response to voltage reference VI*. As a result, system voltage Vs is further increased. In this case, the abnormal voltage of the inverter is detected as in a decrease in system voltage described above, and the operation of the inverter is stopped.
Both the active and reactive powers of the inverter output and the load are rarely the same. However, since an active power reference of the inverter changes along with time, the inverter is involved in one of the above two cases (abnormal frequency or voltage), and is stopped.
As described above, the frequency and voltage of the inverter are changed due to a service interruption, and the abnormal voltage or frequency detector detects this to stop the inverter. Time T from a service interruption to stop of the inverter is required to fall within the range of 0.5 to 1 sec although it is slightly different between Japan and U.S.A. In the conventional apparatus, the following problem is posed.
In order to shorten time T from a service interruption to stop of the inverter, detection sensitivity of the abnormal detector can be improved, or an operation time from detection to operation can be shortened. However, a change in system voltage is presumed to be .+-.5% in a normal state and often be about +10% or -15%. For example, an instantaneous decrease in system voltage upon power-on of a transformer or motor is experienced in a normal state. If the operation level of the abnormal voltage detector is set to fall within the range of +10% and -15%, and the operation time is set to fall within the range of 0.5 to 1 sec, the inverter may be stopped more frequently than needed. For the abnormal frequency detector, it is not preferable to set the detection level to fall within the range of .+-.1 Hz. The operation time of a commercially available abnormal frequency detector is 1 to 2 sec.
Assume that a variation range of the system frequency is 50 Hz .+-.1 Hz, and a frequency equal to or lower than 49 Hz and equal to or higher than 51 Hz is detected as an abnormal frequency. A service interruption occurs while the inverter is operated near 49 Hz, and the oscillation frequency of the inverter is gradually increased and reaches 51 Hz. Thereafter, the abnormal frequency detector is operated after the lapse of 1 sec, and the inverter is stopped. A time required for a variation of 2 Hz from 49 Hz to 51 Hz is about 5 sec under worst conditions. Time T from generation of abnormality to stop of the inverter must be a sum of a 2-Hz frequency change period (5 sec) and an operation time (1 sec) of the abnormal frequency detector, i.e., about 6 sec.
As described above, with the detection technique of the conventional apparatus, it is difficult to stop the inverter within 1 sec.